1. Field of the Invention
The present invention relates to a device for monitoring one or more measurement electrodes and a neutral electrode, attached to a patient for recording physiological measurement signals, and their leads while the measurement signals are being picked up, wherein each measurement electrode is connected to one input socket of an associated measurement amplifier.
2. Description of the Prior Art
In "ECG Electrodes: A Study of Electrical and Mechanical Long-Term Properties", Acta anaesth. Scand. 1979, 23, pp. 189-206, P. Ask et al. describe the measurement of electrode impedances in order to check the quality and adhesion to skin of ECG electrodes. Sinusoidal currents at different frequencies are applied to a pair of ECG electrodes, connected in series, and a reference resistance. Individual electrode impedances are determined from the amplitudes and phases of the ensuing applied voltage and the voltage measured across the resistance.
U.S. Pat. Nos. 4,658,831, 4,917,099 and 4,993,423 describe methods for detecting a detached electrode or interrupted lead line, wherein alternating current at one or more frequencies is applied to the measurement electrodes, and the ensuring voltages are measured. U.S. Pat. Nos. 4,919,145 and 5,020,541 further describe the monitoring of the impedance of ECG electrodes and their leads with the use of two phase-shifted carrier wave signals. These methods fail to supply any absolute value for the individual electrode impedances. Instead, a value is obtained which depends (sum or difference) on the impedances of a number of ECG electrodes, and the impedance of the neutral electrode is not measured. Moreover, is it not possible in these known systems to detect when a specific ECG electrode is in the process of becoming detached. The use of current at different frequencies further leads to an additional measurement difficulty, since electrode impedance varies with frequency.
The use of direct current for detection of electrode detachment or interruption in e.g. electrocardiography is also known. Contact impedance increases if a measurement electrode starts to detach from the patient. Here, an increase in DC potential across the electrode impedance can be achieved with an appropriately arranged resistor. When the potential exceeds a given value, the electrode is no longer viewed as being connected, i.e., a detached lead. The use of direct current for this detection has shortcomings, however, since electrode impedance is not purely resistive. There is some degree of polarization which differs for different types of electrodes. The potential caused by the polarization cannot be distinguished from the potential caused by the detection current through the electrode, so the impedance required to enable detection of any disconnected electrode varies greatly with different types of electrodes.
Another shortcoming is the circumstance that impedance varies whenever the patient moves, voltage across the electrode being modulated in step with the movements, thereby causing noise at about 1 Hz. These baseline variations are hard to filter out of the overall ECG without affecting the ECG signal itself. If possible, non-polarized electrodes, i.e. electrodes with very limited DC offset caused by polarization, are used to avoid baseline variations. The shortcoming here is the need to add a DC potential merely to detect whether or not the electrode is connected.
Reliable monitoring of measurement electrodes attached to the patient is important, so that a warning is issued before the electrode falls off. Electrode impedance increases when the electrode paste starts drying or the electrode begins to detach, thereby distorting the measurement by increasing noise. The measurement signal can also be damped when electrode impedances are very high, possibly causing a faulty diagnosis. Thus, there are e.g. instructions for ensuring a correct absolute value for e.g. ECG signal. Inaccuracies in electrodes leads can cause similar errors.
German OS 41 06 857 describes a device for processing physiological measurement signals, whereby a pulse is delivered to the patient's body via an additional electrode which, via the body's impedance, is electrically connected to measurement electrodes attached to the patient. The measurement electrodes' contact with the patient's body can be checked by studying the measurement electrodes' response to the pulse. This check on the measurement electrodes and their leads, however, is only performed when enabled by the operator, i.e. when she/he presses a button. For continuous monitoring of the electrodes, this function must be supplemented with another method, e.g. the aforementioned DC monitoring.